This invention relates to an image forming apparatus which forms a toner image on a transfer material by transferring a toner image from a photoreceptor element, serving as a photosensitive member, to an intermediate transfer member (hereinafter, also referred to as a intermediate transfer element) and then transferring the toner image from the intermediate transfer member to the transfer sheet.
To assure transferability and cleaning abilities of toners, a conventional image forming apparatus has used a developing agent made of toners and lubricants containing Ca- or Zn-related fatty acid metal salts.
Patent Document 1: Japanese Non-Examined Patent Publication H05-188643.
Patent Document 2: Japanese Non-Examined Patent Publication S60-131547.
Further, the conventional art uses toners, which have a sharp size distribution like polymerized toner and are almost spherical without edges. As these toners are sharp in the size distribution, the charge distribution is also sharp in most cases and their developing properties are good. However, the toners will be easily packed closest, agglutinates, and hard to be transferred. Additionally, such toners are hard to be cleaned. Patent Documents 3 to 6 try to assure transferability and cleaning ability of toners by applying a lubricant to the photoreceptor element besides lubricants added to the toners.
Patent Document 3: Japanese Non-Examined Patent Publication H06-148941.
Patent Document 4: Japanese Non-Examined Patent Publication 2001-282043.
Patent Document 5: Japanese Non-Examined Patent Publication H11-212398.
Patent Document 6: Japanese Non-Examined Patent Publication H11-219087.
However, if the lubricant applied to the photoreceptor element and the lubricant added to the toners are not proper, the photoreceptor element may have a lubricant film on the surface of the photoreceptor element.
Patent Document 7: Japanese Non-Examined Patent Publication 2001-166659.
The lubricant film on the surface of the photoreceptor element is acceptable as far as it is uniform on the whole surface of the photoreceptor element. However, when the photoreceptor element is cleaned by a cleaning blade or the like, some part of the lubricant film may locally remain un-removed on the surface of the photoreceptor element. As the lubricant is high in electric resistance, the total resistance of the local films left on the surface of the photoreceptor element may be high and as the result the transfer ratio reduces. This causes local longitudinal streaks of low or high densities in the image.
Further, naturally, it is impossible to uniformly apply a lubricant to the surface of the photoreceptor element. The lubricant is locally excessive on some part of the surface of the photoreceptor element and insufficient on the other part of the surface. This makes the developing ability and the transferability uneven and finally makes the image rough. In an image forming apparatus that transfers a toner image from a photoreceptor element to an intermediate transfer member and then transfers a toner image from the intermediate transfer member to a transfer sheet, it is impossible to control the formation of a lubricant film on the intermediate transfer member. Consequently, in such an image forming apparatus, the image density may be uneven on the transfer sheet.
Further, the image densities are made uneven also by deterioration of toners in the developing devices. This often occurs when images of low-print areas are formed continuously. This is because most of toners are circulated in the developing devices vainly. As the result, the lubricant added to the toners is lost and the developing ability, transferability, and cleaning ability of the toners are deteriorated.